In Jordan Peele’s incredible horror film from 2019 Us, an army of human doppelgangers called The Tethered arises to take the place of the existing human population. It strikes some of our core fears, that we may actually be the monsters, and that another version of ourselves may rob us of our preferred place on earth. It is later revealed that Tethered are genetic clones created by the government and abandoned.
Human cloning is still beyond our reach. But nature has locked the process. A number of animals, including some reptiles, birds and sharks, clone themselves through asexual reproduction known as parthenogenesis. The elite club of clone animals has a new member.
In recent decades, the planet has been sacrificed to a ten-legged crustacean with many claws and created a clone army that is concerned with world domination. No, it is not an interplanetary intervention or the result of an uncontrolled government experiment. This is biology that has gone wrong, or if you happen to be a marbled crayfish, the biology has gone terribly right.
Today, the freshwater marbled crayfish populate various ecosystems throughout Asia, Europe and Africa, and they all trace back to a single genetically identical individual born less than three decades ago. Their exact population figures are unknown, but there are an estimated 23,000 living in a single small lake in Germany, measuring less than a tenth of a square kilometer, so there is reason for many of them.
Their invasive nature and rapid spread over a significant part of the planet made them an exciting target for scientific research. An international team of researchers completed an analysis of their genome in an attempt to uncover their origins and found that they were strangers than we could have dreamed. Their findings were published in the journal Natural Ecology Evolution.
The genome of the marbled crayfish has 3.5 million base pairs – that is more than the human genome – which consists of about 21,000 genes from 92 chromosomes. What is unusual is that instead of the expected two copies of their chromosomes, marbled crayfish have three. Their genetic makeup is similar to that of the Slough crayfish, a close relative, which led researchers to conclude that the first marbled crayfish was born through an unusual reproductive event when two Slough crayfish mated.
In addition, it seems that the Slough parents come from different parts of the world, which makes it unlikely that they met in nature. Instead, it is believed that they may have been dropped in the same aquarium tank and met in captivity where they would later give birth to their unusual offspring.
It may have been the end of the story, but this new genetic aberration must have somehow escaped the thought – otherwise one of its own cloned offspring did – and gotten out into the wild. It is often believed that clonal species have a higher risk because they lack the genetic diversity that accompanies sexual reproduction, but it has not been a challenge for the marbled crayfish, at least so far.
Despite its unusual origins and asexual reproductive strategy, it has succeeded in getting a sewer all over the world. Although it has not yet appeared in the US wilderness, some areas are taking preventative measures, calling them banned, even in the aquarium trade where they have become popular.
It is a worthy strategy. Once they find their way into an ecosystem, there is probably no stopping them. A single individual can lay 700 eggs, all copies of itself, and they can survive drought conditions by digging into the ground and migrating over land. They are constantly outcompeting and reducing the number of endemic species.
Let’s just hope the marbled crayfish never looks at humanity. If they do, we may never be able to stop them.